The present invention relates to improved methods and solutions for direct copper writing and, particularly, to improved methods and solutions for direct writing of copper for chip modification or other fine structuring purposes.
Copper, because of its high conductivity and resistance to electromigration, is used extensively as an interconnect material in the electronics industry. Recent advances have shown that copper may be deposited advantageously by direct writing. Direct writing or metallization is a method of depositing thin conductive links or traces. A suitable metal-containing precursor, preferably a metalorganic precursor dissolved in solution, is dispensed onto a substrate. The substrate then is dried to remove excess solvent and to uniformly coat the substrate surface with a film having a thickness varying from the submicron to many micron range. The film then is either pyrolized thermally or decomposed photochemically using a laser in air, oxygen, nitrogen, or other suitable atmosphere to convert the metalorganic precursor to its constituent metal elements, oxides or other compounds.
The use of laser direct writing from metalorganic precursor films offers process simplicity and the potential of higher writing speeds compared to gas phase or solid metalorganic deposition techniques, since the ultimate rate of deposition is determined, in many cases, primarily by the kinetics of decomposition and not by mass transport as in gas and liquid phase deposition. High writing speed also can be achieved by using a liquid metalorganic precursor film which directly decomposes to the metal with few evolved volatiles and small activation energy.
The selection of precursors is important to successful direct writing processes. The precursors must meet certain requirements to be of practical use. These include (i) homogeneous film formation, (ii) good adhesion to common substrate materials, (iii) high solubility in a desired solvent or solvent mixture, (iv) high metal content, i.e., sufficient to form continuous metal features after complete decomposition of organic components, (v) low decomposition temperature, and (vi) high optical absorptivity to allow decomposition at a low laser power to prevent substrate damage during the laser writing process.
A preferred precursor recognized in the prior art is copper formate. Copper formate is known to thermally decompose to copper with the evolution of carbon dioxide and hydrogen in a non-oxidizing atmosphere. The fast heating and cooling of the decomposed copper during the writing process results in negligible oxidation after it is formed. In their articles, "Laser Writing of Copper Lines from Metalorganic Films," Appl. Phys. Lett. 51(26), 2254-56 (Dec. 28, 1987) and "Direct Writing of Copper Lines from Copper Formate Film," MRS Proceedings 101, 95-100 (1988), Gupta and Jagannathan describe laser direct writing of copper lines from copper formate films. Lin et al., "Effect of Surfactant on Casting Metalorganic Films and Writing Copper Metal Patterns," J. Mater. Res., Vol. 6, No. 4, 760`65 April 1991), describe the inclusion of a wetting agent and emulsifier in copper formate solutions and the resulting improved results on direct writing. Additional disclosures regarding direct writing utilizing copper formate precursor solutions are found in Hoffmann, et al., "Fast Laser Writing of Copper and Iridium Lines from Thin Solid Surface Layers of Metalorganic Compounds," Applied Surface Science 43, 54-60 (1989) and Gerassimov et al., "Laser-Induced Decomposition of Organometallic Compounds," App. Phys. B28, 266 (1982).
The present inventor also has written on the advantages of laser direct writing of copper from copper formate films. In "Laser Direct Writing of Copper on Various Thin Film Substrate Materials," Applied Surface Science 46, 143-47 (1990), he discusses laser direct writing of copper from dried-on copper formate films on to Al.sub.2 O.sub.3, AlN, polyimide and aluminum. Also, in "YAG Laser Direct Writing of Copper from Copper Formate Films," Appl. Phys. Lett. 56 (10), 904-06 (1990), he discusses depositing very thin copper lines by direct writing techniques using water soluble precursor films from copper formate.
The patent-related literature provides teachings regarding metallization and the use of certain solutions for improved electrical properties. The following references, which in many cases, do not teach copper formate direct writing per se are illustrative of related techniques and solutions. Soviet Union Inventor's Certificate No. 547,462 discloses an adhesive composition, useful for printing plates, electronic components, etc., which includes formic acid and copper salt. Soviet Union Inventor's Certificate No. 1,513,529 discloses a method for producing electroconductive composite materials comprising copper formate, sulphur and a styrene copolymer. European Patent Application No. 368,231 discloses a process for producing a copper-plated resin article utilizing a dry process. The process includes heating a copper formate film under reduced pressure or in a non-oxidizing atmosphere. Research Disclosure No. 29878 discloses a technique for repairing open circuits by applying a doped precursor comprising copper formate and a small amount of polydiacetylene or polythiophene. Soviet Union Inventor's Certificate No. 1,171,511 describes a lubricant for heavy duty machines, which comprises glycerol and a copper salt. Soviet Union Inventor's Certificate No. 1,129,228 discusses the use of copper formate in a lubricant composition. The copper formate purportedly improves the anticorrosion properties of the lubricant. Soviet Union Inventor's Certificate No. 819,148 discloses a light sensitive solution useful for activating dielectric surfaces prior to metallization. The solution comprises copper formate, di-sodium salt of anthraquinone-disulphonic acid, nickel sulfate, potassium bromide, glycerine, saccharose and water. Research Disclosure No. 29367 discloses the localized laser deposition of gold from solid salts. Research Disclosure No. 26162 discusses the accelerated gas phase laser deposition of noble metals. U.S. Pat. No. 3,119,713 discloses a method and apparatus for vapor plating copper from a copper containing composition. The preferred slurry comprises copper formate, formic acid and copper turnings. U.S. Pat. No. 4,007,316 discloses a deferred action battery having an improved depolarizer, which may include copper formate. Finally, Soviet Union Inventor's Certificate No. 730,748 discloses polymer compositions useful for making metallo-polymer articles having good electroconducting mechanical and antifriction characteristics. The composition comprises a phenol-formaldehyde resin or polyvinylfurfural binder, graphite, copper formate and a gas absorber.
The above-described copper formate solutions, and particularly those described in the preceding articles, suffer from a crystallization of the initially amorphous copper formate films upon drying. As the thickness of the applied films increases, the degree of crystallization also increases. This crystallization makes it difficult, if not impossible, to find a useful process window for the laser decomposition of the film due to the different optical absorption properties of the copper formate. This crystallization problem is particularly acute in instances where thicker films are applied to the substrate. Also, many of the currently available solutions do not allow for fine deposition treatment because their lateral resolution capabilities are limited.